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Testing

The primary motivation for creating this library was to help make testing easier when working with SavedStateHandle.getLiveData() and converting this value to a Flow, then to a StateFlow. Without the abstraction SavedStateFlowHandle provides, one would have to use LiveData when testing and remember to always add the InstantTaskExecutorRule. If your app simply uses StateFlow to manage state, then needing to work with LiveData and adding this rule can increase the cognitive load when working in a project.

TestSavedStateFlow

Since the underlying implementation of SavedStateFlow is delegating to the SavedStateHandle, using the actual implementation would require using LiveData and InstantTaskExecutorRule. So to prevent the need of using either of those in tests, there is a test artifact that can be used called TestSavedStateFlow which simply swaps out the implementation with a MutableStateFlow.

So one simple test using setup using TestSavedStateFlow might look like the following. The mocking library in the samples is Mockk. 

class SomeTest {
    @Test
    fun `some test`() = runBlocking {
        val savedStateHandle: SavedStateFlowHandle = mockk()
        val savedStateFlow = TestSavedStateFlow<String>(
            defaultValue = "", 
            cachedValue = "some cached value"
        )
        every { savedStateHandle.getSavedStateFlow(any(), "some-key", "") } returns savedStateFlow

        val viewModel = MyViewModel(savedStateHandle)
        // omitted test code
    }
}

Test Setup

Both samples for manual DI and Hilt have the same constructor, therefore have the same test structure. So when setting up the test, a TestCoroutineTestRule is needed to override the main dispatcher being used by the viewModelScope. Then the following setup will be used to setup each of the following tests one could write using TestSavedStateFlow. 

class MainViewModelTest {

    @get:Rule
    var coroutinesTestRule = TestCoroutinesRule()

    private lateinit var viewModel: MainViewModel

    private val savedStateHandle: SavedStateFlowHandle = mockk()
    private val newsDataSource: NewsDataSource = mockk()

    private val results = listOf<String>("some value", "some second value")

    private fun setUp(savedStateFlow: SavedStateFlow<String>) {
        every { newsDataSource.fetchQuery("") } returns flow {  }
        every { savedStateHandle.getSavedStateFlow(any(), SAVED_STATE_QUERY_KEY, "") } returns savedStateFlow

        viewModel = MainViewModel(savedStateHandle, newsDataSource)
    }

}

Then to actually test the values emitted by the Flow, Turbine is a great testing library for verifying values emitted by a Flow.

Test Default Value

The cachedValue has a default value of null, so that can be omitted in tests not concerned with a cached value when restoring from a process death and just use the defaultValue. 

@Test
fun `default value test`() {
    val savedStateFlow = TestSavedStateFlow<String>("some default value")
    ....
}

Test Cached Value

@Test
fun `initial query value exists, should start in loading state and fetch results`() = runBlocking {
    val cachedQuery = "some cached query"
    val savedStateFlow = TestSavedStateFlow<String>("", cachedQuery)
    every { newsDataSource.fetchQuery(cachedQuery) } returns flowOf(results)

    setUp(savedStateFlow)

    viewModel.state.test {
        assertEquals(State(true, cachedQuery, emptyList()), awaitItem())
        assertEquals(State(false, cachedQuery, results), awaitItem())
    }
}

Test Value Changes

@Test
fun `update query should trigger a new fetch to the repository for results and update state`() = runBlocking {
    val savedStateFlow = TestSavedStateFlow<String>("")
    val newQuery = "some new query"
    every { newsDataSource.fetchQuery(newQuery) } returns flowOf(results)

    setUp(savedStateFlow)

    viewModel.state.test {
        viewModel.updateQuery(newQuery)
        assertEquals(State(false, "", emptyList()), awaitItem())
        assertEquals(State(true, newQuery, emptyList()), awaitItem())
        assertEquals(State(false, newQuery, results), awaitItem())
    }
}

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